Semiconductor device and method of manufacturing the same

ABSTRACT

This invention provides a semiconductor device with a SOI structure and a method of manufacturing the same, preventing deterioration in and making improvement in device characteristics. Nitrogen ion implantation into NMOS and PMOS regions (NR, PR) with resists ( 22   b ) and ( 22   c ) as masks, respectively, introduces nitrogen ions into channel doped layers ( 31 ). The subsequent thermal treatment provides a structure with the channel doped layers ( 31 ) containing nitrogen having a prescribed concentration distribution in the depth direction.

This application is a division of 08/888,898 Jul. 7, 1997 U.S. Pat. No.5,910,672.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to a semiconductor device and a method ofmanufacturing the same, especially to a semiconductor device with a SOI(silicon-on-insulator) structure and its manufacturing method.

2. Description of the Background Art

FIG. 23 is a sectional view showing NMOS and PMOS transistors 24, 25formed on a SOI substrate, as one of examples of the semiconductordevice with the SOI structure.

With reference to FIG. 23, a buried oxide film 2 is formed on the mainsurface of a silicon substrate 1, and a single-crystal silicon layer 3(hereinafter referred to as a SOI layer) is formed on the buried oxidefilm 2. With the SOI layer as a substrate, the NMOS and the PMOStransistors 24, 25 are formed thereon.

The NMOS transistor 24 includes a pair of source/drain layers 35 bindependently formed parallel to each other at the surface of the SOIlayer 3, and a pair of lightly doped drain layers 35 a (hereinafterreferred to as LDD layers) formed adjacent to the facing edges of thesource/drain layers 35 b. A gate oxide film 30 is formed on the SOIlayer 3, and a gate electrode 28 is formed on the gate oxide film 30.Further, sidewall oxide films 26 are formed at the side surfaces of thegate oxide film 30 and the gate electrode 28.

The PMOS transistor 25 includes a pair of source/drain layers 36 bindependently formed parallel to each other at the surface of the SOIlayer 3, and a pair of LDD layers 36 a formed adjacent to the facingedges of the source/drain layers 36 b. The gate oxide film 30 is formedon the SOI layer 3, and the gate electrode 28 is formed on the gateoxide film 30. Further, the sidewall oxide films 26 are formed at theside surfaces of the gate oxide film 30 and the gate electrode 28.

The NMOS and the PMOS transistors 24, 25 are electrically isolated by anisolation oxide film 40 formed so as to reach to the buried oxide film 2from the surface of the SOI layer 3. The isolation oxide film 40isolates the NMOS and the PMOS transistors 24, 25 from other elements aswell.

FIG. 23 also shows that high-concentration impurity regions 130 areformed in contact portions between the isolation oxide film 40 and theSOI layer 3 on which the NMOS transistor 24 is to be formed.

As described above, the NMOS and the PMOS transistors 24, 25 formed onthe SOI substrate have structures with the SOI layer 3, which is to be achannel, held between the gate oxide film 30 and the buried oxide film2. Thus, the SOI layer 3 is inferior in crystalline to a bulk siliconsubstrate, and further is formed thin as is evident from FIG. 23.

Further, into such SOI layer 3, impurity ions are generally implanted ina manufacturing processes, such as channel implantation and source/drainimplantation, of the NMOS and the PMOS transistors 24, 25. This givesdamage to the SOI layer 3 and causes further deterioration incyristalline. Thus, a transistor formed on the SOI substrate is inferiorin transistor characteristics to that formed on the bulk siliconsubstrate.

SUMMARY OF THE INVENTION

A first aspect of the present invention is directed to a semiconductordevice formed on a SOI substrate in which a buried oxide film and a SOIlayer are stacked on a silicon substrate. The semiconductor devicecomprises: a first semiconductor region of a first conductivity typeformed in a predetermined position of the SOI layer, ranging from asurface of the SOI layer to a surface of the buried oxide film; a pairof second semiconductor regions of a second conductivity typeindependently and selectively formed in the surface of the SOI layer soas to sandwich the first semiconductor region; a gate oxide film formedin an upper portion of the first semiconductor region; and a gateelectrode formed on the gate oxide film, wherein the first semiconductorregion contains nitrogen introduced so as to have a predeterminedconcentration distribution in a depth direction thereof, thepredetermined concentration distribution having: a first peak portionprotruding with a first concentration, in the vicinity of an interfacebetween the first semiconductor region and the buried oxide film; and asecond peak portion protruding with a second concentration, in thevicinity of an interface between the first semiconductor region and thegate oxide film.

Preferably, according to a second aspect of the present invention, thepredetermined concentration distribution has a plane portion of nearuniformity with a third concentration lower than the first and thesecond concentrations, in a region held between the first and the secondpeak portions.

Preferably, according to a third aspect of the present invention, thefirst concentration ranges from 1×10¹⁸ to 1×10¹⁹/cm³; the secondconcentration ranges from 1×10¹⁹ to 1×10²⁰/cm³; and the thirdconcentration ranges from 1×10¹⁶ to 1×10¹⁷/cm³.

Preferably, according to a fourth aspect of the present invention, thepredetermined concentration distribution has a third peak portionprotruding with a third concentration at least lower than the secondconcentration, near a center of a region held between the first and thesecond peak portions.

Preferably, according to a fifth aspect of the present invention, thefirst concentration ranges from 1×10¹⁸ to 1×10¹⁹/cm³; the secondconcentration ranges from 1×10¹⁹ to 1×10²⁰/cm³; and the thirdconcentration ranges from 1×10¹⁸ to 5×10¹⁹/cm³.

Preferably, according to a sixth aspect of the present invention, thesecond semiconductor region contains nitrogen introduced so as to havenearly the same concentration with the second concentration.

A seventh aspect of the present invention is directed to a method ofmanufacturing a semiconductor device formed on a SOI substrate in whicha buried oxide film and a SOI layer are stacked on a silicon substrate.The method of manufacturing the semiconductor device comprises the stepsof: (a) preparing the SOI substrate; (b) defining a device formingregion for forming the semiconductor device by electrically isolating apredetermined region of the SOI layer from other regions; (c) forming afirst semiconductor region of a first conductivity type byion-implanting an impurity of a first conductivity type and nitrogeninto the device forming region; (d) giving heat treatment to the firstsemiconductor region in such a condition that the nitrogen gets apredetermined concentration distribution in a depth direction of thefirst semiconductor region; (e) forming a gate oxide film on the firstsemiconductor region; (f) forming a gate electrode on the gate oxidefilm; and (g) forming second semiconductor regions of a secondconductivity type by ion-implanting an impurity of a second conductivitytype and nitrogen into the first semiconductor region with the gateelectrode as a mask, wherein the predetermined concentrationdistribution has: a first peak portion protruding with a firstconcentration, in the vicinity of an interface between the firstsemiconductor region and the buried oxide film; and a second peakportion protruding with a second 6 concentration, in the vicinity of aninterface between the first semiconductor region and the gate oxidefilm.

Preferably, according to an eighth aspect of the present invention, thestep (c) comprises a step of ion-implanting the nitrogen afterion-implanting the impurity of a first conductivity type.

Preferably, according to a ninth aspect of the present invention, thestep (c) comprises a step of ion-implanting the impurity of a firstconductivity type after ion-implanting the nitrogen.

Preferably, according to a tenth aspect of the present invention, thenitrogen is implanted at a dose of 0.1×10¹² to 100×10¹²/cm² at an energyof 20 to 35 keV.

Preferably, according to an eleventh aspect of the present invention,the step (d) comprises a step of performing annealing in nitrogenatmosphere for 5 to 30 minutes at a temperature of 800 to 900° C.

Preferably, according to a twelfth aspect of the present invention, thestep (e) comprises a step of forming the gate oxide film by thermaloxidation; and the heat treatment in the step (d) may also serve as thestep of forming the oxide film by thermal oxidation in the step (e).

Preferably, according to a thirteenth aspect of the present invention,the step (g) comprises the steps of: (g-1) ion-implanting the impurityof a second conductivity type into the first semiconductor region withthe gate electrode as a mask; and (g-2) forming an oxide film on thegate oxide film and the gate electrode, and then ion-implanting thenitrogen through the oxide film.

Preferably, according to a fourteenth aspect of the present invention,the step (g-2) comprises a step of ion-implanting the nitrogen at a doseof 0.1×10¹² to 10×10¹²/cm² at an energy of 5 to 20 kev.

Preferably, according to a fifteenth aspect of the present invention,the step (f) comprises the steps of: (f-1) forming a polysilicon layeron the gate oxide film; and (f-2) forming the gate electrode byselectively removing the polysilicon layer after ion-implanting nitrogeninto the polysilicon layer.

Preferably, according to a sixteenth aspect of the present invention,the step (f-2) comprises a step of implanting the nitrogen at a dose of3×10¹⁴ to 12×10¹⁴/cm² at an energy of 5 to 30 keV.

In the semiconductor device of the first aspect of the presentinvention, the first semiconductor region contain nitrogen introduced soas to have the concentration distribution in the depth direction, theconcentration distribution having the first peak portion protruding withthe first concentration, in the vicinity of the interface between thefirst semiconductor region and the buried oxide film; and the secondpeak portion protruding with the second concentration, in the vicinityof the interface between the first semiconductor region and the gateoxide film. Thus, deterioration in transistor characteristics can beprevented by combining nitrogen with dangling bonds existing a lot inthe interface between the first semiconductor region and the buriedoxide film, while transistor characteristics can be improved bycombining nitrogen with dangling bonds existing a lot in the interfacebetween the first semiconductor region and the gate oxide film.

In the semiconductor device of the second aspect of the presentinvention, nitrogen is almost uniformly distributed with the thirdconcentration lower than the first and the second concentration in aregion between the first and the second peak portions. The nitrogen istrapped in the crystal defects in the SOI layer, preventing an impurityfrom being trapped in the crystal defects. This prevents variations inconcentration of the impurity active in the SOI layer. Thus,deterioration in transistor characteristics caused by existence of thecrystal defects can be prevented, and further similar transistorcharacteristics to the bulk transistor become available in the SOItransistor.

In the semiconductor device of the third aspect of the presentinvention, optimum value in the concentration distribution of nitrogenachieves a practical semiconductor device preventing deterioration inand making improvement in transistor characteristics.

In the semiconductor device of the fourth aspect of the presentinvention, nitrogen concentration distribution has the third peakportion protruding with the third concentration lower than the first andthe second concentration, in a portion between the first and the secondpeak portions. The nitrogen is trapped in the crystal defects in the SOIlayer, preventing an impurity from being trapped in the crystal defects.This prevents vacations in concentration of the impurity active in theSOI layer. Thus, deterioration in transistor characteristics caused byexistence of the crystal defects can be prevented, and further similartransistor characteristics to the bulk transistor become available inthe SOI transistor.

In the semiconductor device of the fifth aspect of the presentinvention, optimum value in the concentration distribution of nitrogenachieves a practical semiconductor device preventing deterioration inand making improvement in transistor characteristics.

In the semiconductor device of the sixth aspect of the presentinvention, nitrogen of nearly the same concentration with that in thesecond peak portion is introduced in the second semiconductor regions(i.e., source/drain layers). Thus, impurity concentration in the secondsemiconductor regions becomes higher than that in the firstsemiconductor region, which makes it possible to cope with a lot ofcrystal defects due to impurity implantation. This achieves asemiconductor device which prevents deterioration in and makesimprovement in transistor characteristics.

The seventh aspect of the present invention provides the method ofmanufacturing a semiconductor device desirable for the semiconductordevice of the first aspect of the present invention.

In the method of manufacturing the semiconductor device of the eighthaspect of the present invention, nitrogen is trapped in the crystaldefects with subsequent thermal treatment by implanting nitrogen afterimplanting the impurity of a first conductivity type.

In the method of manufacturing the semiconductor device of the ninthaspect of the present invention, nitrogen is trapped in the crystaldefects with subsequent thermal treatment by implanting the impurity ofa first conductivity type after implanting nitrogen ions.

In the method of manufacturing the semiconductor device of the tenthaspect of the present invention, it is possible to obtain optimumconditions for implanting nitrogen ions into the first semiconductorregions.

In the method of manufacturing the semiconductor device of the eleventhaspect of the present invention, it is possible to obtain optimumconditions for diffusing nitrogen ions implanted into the firstsemiconductor region so that the nitrogen ions have a prescribedconcentration distribution in the depth direction.

In the method of manufacturing the semiconductor device of the twelfthaspect of the present invention, the step of diffusing nitrogen ionsimplanted into the first semiconductor regions serves also as that offorming the gate oxide films, which suppresses increase in the number ofthe steps in the preferred embodiments of the present invention.

In the method of manufacturing the semiconductor device of thethirteenth aspect of the present invention, nitrogen is trapped in thecrystal defects due to impurity implantation by implanting nitrogen informing the second semiconductor region as well, which preventsdeterioration in transistor characteristics. Further, nitrogen ionimplantation performed through the oxide film in this case preventsdeterioration in crystalline of the SOI layer caused by the nitrogen ionimplantation.

In the method of manufacturing the semiconductor deceive of thefourteenth aspect of the present invention, it is possible to obtainoptimum conditions for implanting nitrogen ions into the secondsemiconductor regions.

In the method of manufacturing the semiconductor device of the fifteenthaspect of the present invention, nitrogen ion implantation into the gateelectrodes introduces nitrogen to the gate oxide films as well, therebyincreasing nitrogen concentration in the vicinity of the interfacebetween the first semiconductor regions and the gate oxide films.

In the method of manufacturing the semiconductor device of the sixteenthaspect of the present invention, it is possible to obtain optimumconditions for implanting nitrogen ions into the gate electrodes.

This invention provides a semiconductor device with the SOI structurewhich prevents deterioration in and makes improvement in devicecharacteristics, and its manufacturing method.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 17 are sectional views illustrating a manufacturing processof a semiconductor device in accordance with preferred embodiments ofthe present invention.

FIG. 18 is a partly sectional view showing a structure of thesemiconductor device in accordance with the preferred embodiment of thepresent invention.

FIG. 19 is a graph illustrating concentration distribution of nitrogenin the semiconductor device in a horizontal sectional direction inaccordance with the preferred embodiment of the present invention.

FIGS. 20 and 21 are graph illustrating concentration distribution ofnitrogen in the semiconductor device in a vertical sectional directionin accordance with the preferred embodiments of the present invention.

FIG. 22 is a sectional view showing a process of nitrogen ionimplantation into LDD layers through no oxide film.

FIG. 23 is a sectional view showing a structure of a SOIMOS transistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<1. Manufacturing Process>

A process of forming a MOS transistor on a SOI substrate in accordancewith the present invention is now explained with reference to FIGS. 1 to17.

FIGS. 1 to 17 are sectional views showing in sequence the process offorming NMOS and PMOS transistors on the SOI substrate.

First, a SOI substrate 10 as shown in FIG. 1 is prepared. The SOIsubstrate 10 has a buried oxide film 2 and a single-crystal siliconlayer 3 (hereinafter referred to as a SOI layer) stacked one above theother on a silicon substrate 1. A method of manufacturing the SOIsubstrate 10 may be a SIMOX (separation by implanted oxygen) method or abonding method.

With reference to FIG. 2, an oxide film 201 with a thickness of 100 to300 A is, for example, formed on the SOI layer 3 at temperature near800° C. by a CVD (chemical vapor deposition) method. The oxide film 201may be formed by thermal oxidation at temperature near 800° C. Then, annitride film 211 with a thickness of 1000 to 2000 A is formed on theoxide film 201 at temperature near 700° C.

The nitride film 211 is selectively removed so as to leave a nitridefilm 21 only on a region to be active in the SOI layer 3 by means of,for example, dry etching. Then, with reference to FIG. 3, a resist 22 ais formed on a region PR (hereinafter referred to as a PMOS region)where the PMOS transistor is to be formed. With the resist 22 a as amask, high-concentration impurity regions (P⁺) 13 are formed in a regionNR (hereinafter referred to as an NMOS region) where the NMOS transistoris to be formed, for example, by implanting boron ions from a diagonaldirection at a dose of 3×10¹³ to 8×10¹³/cm² at an energy of 20 to 35keV. The implantation is performed at an angle of about 45 degree withrotation of the SOI substrate 10.

Implanting boron ions from a diagonal direction with rotation of the SOIsubstrate 10, the high-concentration impurity regions 13 is also formedin the SOI layer 3 under the nitrogen film 21 or the resist 22 a. Thisconstruction rises a threshold voltage of a parasitic transistor formedin a portion to be the edge of the SOI layer 3, thereby preventingactivation of the parasitic transistor.

Further, annealing is performed for about 2 to 5 seconds at atemperature of 750 to 950° C. after the ion implantation shown in FIG.3. This recovers crystalline of the SO layer 3 damaged by the ionimplantation.

Next, the resist 22 a is removed, and, as shown in FIG. 4, an isolationoxide film 4 is selectively formed by LOCOS oxidation with the nitridefilm 21 as a mask to provide an active region in the SOI layer 3. Then,the PMOS and the NMOS regions PR, NR, which are electrically isolatedfrom each other by the buried oxide film 2 and the isolation oxide film4, are formed by removing the nitride films 21 with thermal phosphoricacid.

Next, with reference to FIG. 6, a resist 22 b is formed on the PMOSregion PR. With the resist 22 b as a mask, impurity ions are implantedin the NMOS region NR. This implantation makes the SOI layer 3 in theNMOS region NR, of a channel doped layer 31 (first semiconductorregion).

In this ion implantation, boron ions (B) are, for example, implanted ata dose of 3×10¹² to 8×10¹²/cm² at an energy of 20 to 35 keV.

After that, with the resist 22 b as a mask, nitrogen ions are implantedinto the NMOS region NR at a dose of 0.1×10¹² to 100×10¹²/cm² at anenergy of 20 to 35 keV.

Then, the resist 22 b is removed, and a resist 22 c is formed on theNMOS region NR as shown in FIG. 7. With the resist 22 b as a mask,impurity ions are implanted into the PMOS region PR. This implantationmakes the SOI layer 3 in the PMOS region, of the channel doped layer 31.

In this ion implantation, phosphorous ions (P) are, for example,implanted at a dose of 3×10¹² to 8×10¹²/cm² at an energy of 20 to 60keV.

After that, with the resist 22 c as a mask, nitrogen ions (N) areimplanted into the PMOS region PR at a dose of 0.1×10¹² to 100×10¹²/cm²at an energy of 20 to 35 keV.

Next, the SOI substrate with the NMOS and the PMOS regions NR, PR wherenitrogen ions are implanted is exposed to nitrogen atmosphere, andannealed for about 5 to 30 minutes at temperature near 820° C.

<1-1. First Effect of Nitrogen Ion Implantation>

The following description explains the reason for implanting nitrogenions into the PMOS and the NMOS regions PR, NR.

In general, crystal defects are distributed over the SOI layer indensity of 1×10² to 1×10⁷/cm². If impurity ions are implanted into suchSOI layer, some of them may be diffused and be trapped in the crystaldefects by subsequent heat treatment. The more crystal defects exist,the more active impurity ions are reduced in the vicinity of the crystaldefects, which results in deterioration in transistor characteristics.This would not occur in a transistor formed in a bulk silicon layer(hereinafter referred to as a bulk transistor) which is superior incrystalline or in a transistor formed in a polysilicon layer (e.g., thinfilm transistor).

On the other hand, if nitrogen ions are simultaneously implanted withimpurity ions, nitrogen ions are trapped instead of the impurity ions.Thus, effective crystal defects (crystal defects which can trap atoms)are decreased, thereby preventing reduction in active impurity ions inthe vicinity of the crystal defects. That is, deterioration intransistor characteristics caused by the crystal defects can beprevented and similar transistor characteristics to the bulk transistorcan be obtained in the SOI transistor.

Further, nitrogen has the property of substitutional diffusing likeboron with diffusion coefficient larger than boron. Thus, nitrogen mightbe trapped in the crystal defects prior to boron. The diffusion in adifferent form, however, would not affect the effect as described above.

Furthermore, though density of the crystal defects differs among eachSOI substrate, reduction in effective crystal defects as described aboveavoids a possibility that each SOI substrate may have different impurityconcentration. Thus, variations in transistor characteristics betweenlots and between batches of the SOI substrate can be reduced in massproduction of semiconductor devices.

When a dose of nitrogen ion implantation exceeds 100×10¹²/cm², forexample, around 1×10¹⁵/cm², the SOI layer is damaged by nitrogen ions.In this case, nitrogen ion implantation causes deterioration intransistor characteristics rather than improving its characteristics.

Further, viewed in the fact that nitrogen ions are used to be trapped inthe crystal defects, nitrogen ion implantation may be performed beforeimpurity ion implantation.

Moreover, though nitrogen ions are separately implanted into the NMOSand the PMOS regions NR, PR in the process shown in FIGS. 6 and 7, thismay be simultaneously performed either after or before impurity ionimplantation into the NMOS and the PMOS regions NR, PR. This reduces thenumber of switching ions between nitrogen and impurity, thereby reducingloss of time in switching of ion-species and improving productionefficiency.

Further, an implant energy of boron and phosphorous ions, as describedabove with FIGS. 6 and 7, is set up so that the center of the SOI layer3 in the depth direction may be the peak of impurity distribution, whenthe SOI layer 3 is about 1000 angstrom in thickness. Thus, the implantenergy would differ depending on species of impurities or a thickness ofthe SOI layer 3.

<1-2. Second Effect of Nitrogen Ion Implantation>

The following description is about another effect of nitrogen ionimplantation.

As will be described later with a completed structure shown in FIG. 17,the NMOS and the PMOS transistors 14, 15 have the structure with thechannel doped layer 31, which is to be a channel, held between the gateoxide film 20 and the buried oxide film 2. In general, there are a lotof dangling bonds caused by crystal defects in an interface between asilicon layer and an oxide film, which is one of the causes ofdeterioration in transistor characteristics. The SOI transistorespecially includes two interfaces between the silicon layer and theoxide film: the interface between the channel doped layer 31 and theburied oxide film 2 (hereinafter referred to as a first interface); andthe interface between the channel doped layer 31 and the gate oxide film20 (hereinafter referred to as a second interface).

If the first interface is in a bad condition (i.e., a lot of danglingbonds), it is difficult to obtain a prescribed transistorcharacteristics, and only a transistor inferior in transistorcharacteristics to the bulk transistor is available. Since the conditionof the first interface is worse than that of the second interface, it isimportant to improve the condition of the first interface in order toprevent deterioration in transistor characteristics.

When the second interface is in a bad condition (i.e., a lot of danglingbonds), improvement in transistor characteristics including hot carrierresistance is not attainable.

Inactivation of dangling bonds by combining them with implanted nitrogenions, however, would prevent deterioration in and make improvement intransistor characteristics.

In order to inactivate dangling bonds, it becomes important to segregatenitrogen ions in the vicinity of the first and the second interfacesbesides diffusing and trapping them in crystal defects in the channeldoped layer 3 as described in the foregoing. The annealing conditionspreviously described (i.e., nitrogen atmosphere, temperature of about820° C., and time of 5 to 30 minutes) is obtained by experimentsconducted by the inventors, meeting the requirement of the diffusingconditions of nitrogen ions.

The annealing temperature may be near 800 to 900° C. and the annealingtime may be more than 30 minutes. Too high annealing temperature or toolong annealing time, however, causes excessive diffusion of nitrogenions, preventing segregation of nitrogen ions in the vicinity of thefirst and the second interfaces.

Now, getting back to the manufacturing process continued from FIG. 7,the resist 22 c and the gate oxide film 201 are removed after theprocess shown in FIG. 7, and more gate oxidation is performed to form agate oxide film 20 on the surface of the channel doped layer 31 (notshown).

Next, with reference to FIG. 8, a gate polysilicon layer 81 to be gateelectrodes is formed on the NMOS and the PMOS regions NR, PR, forexample, with a thickness of 1000 to 3000 angstrom by a CVD method.Then, a resist 22 d is formed on the NMOS region NR to implant impurityions into the polysilicon layer 81 on the PMOS region PR. In this ionimplantation, boron ions are, for example, implanted at a dose of 3×10¹⁵to 8×10¹⁵/cm² at an energy of 5 to 20 keV. After that, with the resist22 d as a mask, nitrogen ions are implanted in the polysilicon layer 81on the PMOS region PR at a dose of 3×10¹⁴ to 12×10¹⁴/cm² at an energy of5 to 30 keV.

After the resist 22 d is removed, a resist 22 e is formed on the PMOSregion PR to implant impurity ions into the polysilicon layer 81 on theNMOS region NR, as shown in FIG. 9. In this ion implantation, arsenicions (AS) are, for example, implanted at a dose of 3×10¹⁵ to 8×10¹⁵/cm²at an energy of 5 to 20 keV.

Then, with the resist 22 e as a mask, nitrogen ions are implanted intothe NMOS region NR at a dose of 3×10¹⁴ to 12×10¹⁴/cm² at an energy of 5to 30 keV.

<1-3. Effect of Nitrogen Implantation into Gate Electrode>

The impurity ion implantation into the polysilicon layer 81 is anecessary measure to control work functions in using the polysiliconlayer 81 as gate electrodes. Further, the reason why each implant energyof boron ions and arsenic ions is the same is that those ions areimplanted only into the top surface of the polysilicon layer 81. Theimpurity ions are diffused by subsequent heat treatment and are almostuniformly distributed in the polysilicon layer 81.

Moreover, nitrogen ion implantation into the polysilicon layer 81 aimsto inactivate dangling bonds in the interface (second interface) betweenthe oxide film 20 and the channel doped layer 31 by segregating nitrogenin the vicinity of the gate oxide film 20. Though the nitrogen ionimplantation into the SOI layer as shown in FIGS. 6 and 7 and thesubsequent annealing process have already inactivated most of thedangling bonds in the second interface, further nitrogen ionimplantation into the polysilicon layer 81 makes almost all danglingbonds in the second interface inactive, thereby bringing about furtherimprovements in transistor characteristics.

Though a p-type impurity (boron) is introduced in the polysilicon layer81 on the PMOS region PR, existence of nitrogen ions prevents diffusionof the p-type impurity in the channel doped layer 31 through the gateoxide film 20.

Now, getting back to the manufacturing process continued from FIG. 9,after the process shown in FIG. 9, the polysilicon layer 81 isselectively removed to form a gate electrode 8 a in a prescribedposition on the oxide film 20 of the NMOS region NR, and a gateelectrode 8 b in a prescribed position on the oxide film 20 of the PMOSregion PR.

Then, a resist 22 f is formed on the NMOS region NR as shown in FIG. 10.With the resist 22 f and the gate electrode 8 b as masks, ionimplantation into the PMOS region PR is performed to form lightly dopeddrain layers 6 a (hereinafter referred to as LDD layers) in the channeldoped layer 31.

In this ion implantation, boron ions are, for example, implanted at adose of 0.1×10¹² to 10×10¹²/cm² at an energy of 5 to 20 keV.

Next, with reference to FIG. 11, a resist 22 g is formed on the PMOSregion PR. With the resist 22 g and the gate electrode 8 a as masks, ionimplantation into the NMOS region NR is performed to form LDD layers 5 ain the channel doped layer 31.

In this ion implantation, arsenic ions are, for example, implanted at adose of 0.1×10¹² to 10×10¹²/cm² at an energy of 5 to 20 keV.

After this implantation, annealing may be performed for a severalminutes (about 2 to 5 minutes) at a temperature of 750 to 850° C. inorder to recover crystalline.

Next, with reference to FIG. 12, an oxide film 50 with a thickness ofabout 50 to 200 angstrom is formed in the upper portion of the NMOS andthe PMOS regions NR, PR.

After that, nitrogen ions are implanted into the LDD layers 5 a and 5 bof the NMOS and the PMOS regions NR, PR, at a dose of 0.1×10¹² to10×10¹²/cm² at an energy of 5 to 20 keV.

<1-4. First Effect of Nitrogen Ion Implantation through Oxide Film>

The object of nitrogen ion implantation into the LDD layers 5 a and 6 ais to prevent deterioration in transistor characteristics by trappingnitrogen in crystal defects caused by impurity ion implantation. Thereason why nitrogen ion implantation is performed through the oxide film50 is that it prevents deterioration in crystalline in the channel dopedlayer 31. That is, nitrogen ion implantation is performed in order toprevent deterioration in and make improvement in transistorcharacteristics. In some cases, however, nitrogen ion implantationitself may cause deterioration in crystalline in the channel doped layer31. Though crystalline is recovered by annealing in such cases, severedamage on crystalline due to too much implantation would not besufficiently recovered.

When nitrogen ions are implanted through an oxide film, however,deterioration in crystalline due to the implantation is relieved,thereby increasing the effect of annealing on recovery of crystalline.

Further, though nitrogen ions are implanted simultaneously into the LDDlayers 5 a and 6 a as shown in FIG. 12, the ion implantation may beseparately performed. In such case, it is possible to independentlyprovide implant conditions, so nitrogen ion implantation into the NMOSand the PMOS regions NR, PR can be performed under optimum conditions,respectively.

For example, the optimum dose for the LDD layers 5 a (i.e., source/drainlayers) of the NMOS transistor ranges from 1×10¹² to 5×10¹²/cm² whilethat for the LDD layers 6 a (i.e., source/drain layers) of the PMOStransistor ranges from 5×10¹² to 10×10¹²/cm². The implant energy in eachcase is about 20 keV.

Now, getting back to the manufacturing process continued from FIG. 12,after the whole oxide film 50, and the gate oxide film 20 exceptportions directly under the gate electrodes 8 a and 8 b are removed, anoxide film 161 with a thickness of about 1000 to 2000 angstrom is formedin the upper portion of the NMOS and the PMOS regions NR, PR. This oxidefilm 161 may be formed of a TEOS oxide film made from TEOS (tetraethylorthosilicate). Alternatively, the oxide film 161 may be formed withoutremoving the gate oxide film 20.

Then, anisotropic etching to the oxide film 161 provides sidewall oxidefilms 16 as shown in FIG. 14, leaving the oxide film 161 only at theside surfaces of the gate electrodes 8 a, 8 b and respective gate oxidefilms 20.

Next, with reference to FIG. 15, a resist 22 h is formed on the PMOSregion PR. With the resist 22 h, the gate electrode 8 a and the sidewalloxide films 16 as masks, source/drain layers 5 b are formed in thechannel doped layer 31 by ion implantation into the NMOS region NR.

In this ion implantation, arsenic ions are, for example, implanted at adose of 1×10¹⁴ to 50×10¹⁴/cm² at an energy of 5 to 20 keV

After the resist 22 h is removed, a resist 22 i is formed on the NMOSregion NR as shown in FIG. 16. With the resist 22 i, the gate electrode8 b and the sidewall oxide films 16 as masks, source/drain layers 6 bare formed in the channel doped layers 31 by ion implantation into thePMOS region PR.

In this ion implantation, boron ions are, for example, implanted at adose of 1×10¹⁴ to 50×10¹⁴/cm² at an energy of 5 to 20 keV.

After that, interlayer insulation films 60 are formed on the NMOS andthe PMOS regions NR, PR. Then, contact holes CH are formed so as toreach to the source/drain layers 5 b and 6 b from the main surfaces ofthe interlayer insulation films 60, respectively. Finally, wiring layersHL are formed of metal compounds such as aluminum compound in thecontact holes CH to complete the NMOS and the PMOS transistors 14 and 15as shown in FIG. 17.

FIG. 18 shows the structure of the NMOS transistor 14. FIGS. 19 and 20show concentration distribution of nitrogen in a sectional directiontaken along the lines B—B, and C—C of FIG. 18, respectively.

In FIG. 19, the horizontal axes represents a position taken along theline B—B, and the vertical axes represents nitrogen concentration(cm⁻³). As shown in FIG. 19, nitrogen concentration in the channel dopedlayer 31 positioned under the gate electrode 8 a (i.e., channel regionsCR) is about 1×10¹⁷/cm³ while that in the source/drain layers 5 b rangesfrom 1×10¹⁹ to 1×10²⁰/cm³.

In FIG. 20, the vertical axes represents nitrogen concentration (cm⁻³),and the horizontal axes represents a position taken along the line C—C(i.e., position in the depth direction). Reference characters GR, OX, SRand BOR represent a gate region, a gate oxide film region, a SOI region,and a buried oxide film region, respectively, from the left.

As shown in FIG. 20, in the SOI region SR, nitrogen concentration in thevicinity of the gate oxide film region OX is about 1×10²⁰/cm³ and thatin the vicinity of the buried oxide film region BOR is about 1×10¹⁹/cm³,both of which are prominent among nitrogen concentration in the otherregions (about 1×10¹⁷/cm³). This clearly explains, as previouslydescribed, that nitrogen is segregated in the interfaces between thechannel doped layer 31 and the buried oxide film 2 (first interface) andbetween the channel doped layer 31 and the gate oxide film 20 (secondinterface).

Further, in FIG. 20, nitrogen is uniformly distributed in the SOI regionSR except in the edge portions thereof. The implanted nitrogen ions,however, may not be uniformly diffused depending on the conditions ofthermal treatment after the nitrogen ion implantation. Such state isshown in FIG. 21.

In FIG. 21, concentration distribution of nitrogen has one of peaks inthe center portion of the SOI region SR. Though the nitrogen ions areimplanted so as to have a peak of its concentration near the centerportion of the SOI layer 3 and are diffused by thermal treatment, if thethermal treatment are performed under undesirable conditions such as lowtemperature or short period, the peak remains in the center portion ofthe channel doped layer 31.

In such case that nitrogen concentration is high in the center portionof the channel doped layer 31, if nitrogen is segregated in the firstand the second interfaces, however, it is possible to preventdeterioration in and make improvement in transistor characteristics.

<1-5. Second Effect of Nitrogen Ion Implantation through Oxide Film>

The following description gives another effect of nitrogen ionimplantation into the LDD layers 5 a and 6 a through the oxide film 50,in addition to prevention of deterioration in crystalline of the channeldoped layer 31 described above with reference to FIG. 12.

FIG. 22 is a partly sectional view in implanting impurity ions with nooxide film 50 on the upper portions of the NMOS and PMOS regions NR, PR.

As shown in FIG. 22, a channel doped layer CD where impurity ions areimplanted in the SOI layer, gate oxide films GO and a gate electrode GEare formed one above the other on the upper portion of the buried oxidefilm BO. With the gate electrode GE as a mask, nitrogen ions areimplanted into LDD layers LD in the channel doped layer CD. Nitrogen ionimplantation into such structure is likely to reduce reliability in thegate oxide film GO, depending on its dose.

More specifically, regions (shown by X in the figure) in the vicinity ofthe edge end portion of the gate electrode GE in the gate oxide films GOmay be damaged by nitrogen ion implantation, thereby sufferingdeterioration in electrical insulation of the gate oxide film GO.Further, though entirely damaged, the gate oxide film GO except aportion under the gate oxide electrode GE will be removed. Thus, thedamage is not worth of considering.

Further, covering the gate electrode with the oxide film protects thegate oxide film GO in the vicinity of the gate electrode GE, which wouldnot cause any damage to the region X.

As described above and shown in FIG. 12, nitrogen ion implantation intothe LDD layers 5 a and 6 a performed through the oxide film 50 preventsdamage to the gate oxide film 20 due to implantation, and furtherprevents deterioration in reliability of the gate oxide film 20.

<2. Modification>

In the above-described preferred embodiment, annealing is performed inorder to diffuse nitrogen ions after nitrogen ions are implanted into achannel doped layer as shown in FIGS. 6 and 7. Alternatively, thenitrogen ions may be diffused by making use of gate oxidation performedafter nitrogen ion implantation into the channel doped layer.

More specifically, if gate oxidation is performed by thermal oxidation,the channel doped layer 31 is heated as well, thereby diffusingimplanted nitrogen ions. Further, though conditions necessary for thegate oxidation and that necessary for the diffusion of nitrogen ionshave to meet with each other in this case, the present manufacturingprocess would make it possible.

Further, if all conditions are met, the annealing described above mayalso serve as other heating process, which may not be gate oxidationprocess, such as annealing after ion implantation to form LDD layers, ifit fits all the conditions.

Furthermore, though a MOSFET is used as an example of semiconductordevices formed on the SOI substrate in the preferred embodiments andmodification of the present invention, the effect is not limited only tothe MOSFET. The same effect of the present invention would be availablein any semiconductor devices formed on the SOI substrate and having apossibility that an implanted impurity may be trapped by dangling bonds.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

We claim:
 1. A method of manufacturing a semiconductor device formed ona SOI substrate in which a buried oxide film and a SOI layer are stackedon a silicon substrate, said method comprising the steps of: (a)preparing said SOI substrate; (b) defining a device forming region forforming said semiconductor device by electrically isolating apredetermined region of said SOI layer from other regions; (c) forming afirst semiconductor region of a first conductivity type byion-implanting an impurity of a first conductivity type and nitrogeninto said device forming region; (d) giving heat treatment to said firstsemiconductor region in such a condition that said nitrogen gets apredetermined concentration distribution in a depth direction of saidfirst semiconductor region; (e) forming a gate oxide film on said firstsemiconductor region; (f) forming a gate electrode on said gate oxidefilm; and (g) forming second semiconductor regions of a secondconductivity type by ion-implanting an impurity of a second conductivitytype and nitrogen into said first semiconductor region with said gateelectrode as a mask, wherein said predetermined concentrationdistribution has: a first peak portion protruding with a firstconcentration, in the vicinity of an interface between said firstsemiconductor region and said buried oxide film; and a second peakportion protruding with a second concentration, in the vicinity of aninterface between said first semiconductor region and said gate oxidefilm.
 2. The method of manufacturing a semiconductor device according toclaim 1, wherein said step (c) comprises a step of ion-implanting saidnitrogen after ion-implanting said impurity of a first conductivitytype.
 3. The method of manufacturing a semiconductor device according toclaim 2, wherein said nitrogen is implanted at a dose of 0.1×10¹² to100×10¹²/cm² at an energy of 20 to 35 keV.
 4. The method ofmanufacturing a semiconductor device according to claim 1, wherein saidstep (c) comprises a step of ion-implanting said impurity of a firstconductivity type after ion-implanting said nitrogen.
 5. The method ofmanufacturing a semiconductor device according to claim 4, wherein saidnitrogen is implanted at a dose of 0.1×10¹² to 100×10¹²/cm² at an energyof 20 to 35 keV.
 6. The method of manufacturing a semiconductor deviceaccording to claim 1, wherein said step (d) comprises a step ofperforming annealing in nitrogen atmosphere for 5 to 30 minutes at atemperature of 800 to 900° C.
 7. The method of manufacturing asemiconductor device according to claim 1, wherein said step (e)comprises a step of forming said gate oxide film by thermal oxidation;and said heat treatment in said step (d) may also serve as said step offorming said oxide film by thermal oxidation in said step (e).
 8. Themethod of manufacturing a semiconductor device according to claim 1,wherein said step (g) comprises the steps of: (g-1) ion-implanting saidimpurity of a second conductivity type into said first semiconductorregion with said gate electrode as a mask; and (g-2) forming an oxidefilm on said gate oxide film and said gate electrode, and thenion-implanting said nitrogen through said oxide film.
 9. The method ofmanufacturing a semiconductor device according to claim 8, wherein saidstep (g-2) comprises a step of ion-implanting said nitrogen at a dose of0.1×10¹² to 10×10¹²/cm² at an energy of 5 to 20 keV .
 10. The method ofmanufacturing a semiconductor device according to claim 1, wherein saidstep (f) comprises the steps of: (f-1) forming a polysilicon layer onsaid gate oxide film; and (f-2) forming said gate electrode byselectively removing said polysilicon layer after ion-implantingnitrogen into said polysilicon layer.
 11. The method of manufacturing asemiconductor device according to claim 10, wherein said step (f-2)comprises a step of implanting said nitrogen at a dose of 3×10¹⁴ to12×10¹⁴/cm² at an energy of 5 to 30 keV.